In many applications, it is required to focus on an interface rather than on a surface. This is the case whenever the preparation is protected or held by a transparent medium and is to be observed through the transparent medium. Examples are cover slips in microscopy, the specimen carrier glass when the specimen is to be observed from below, and specimen vessels such as microtiter plates or cuvettes in which the preparation to be examined collects on the transparent bottom or cuvette wall.
When the surface to be observed is very large, not only is it necessary to focus the specimen in the center of the visual field, but the plane to be observed must also be oriented with respect to its inclination so that the entire object plane is imaged sharply. The inclination must be oriented with respect the optical axis of the objective used for observation when the objective comprehends the observed surface in its entirety, or with respect to the plane in which the specimen is moved when the surface is scanned, so that it may be detected in its entirety.
In the case with which the invention is concerned, molecules are bound to a glass substrate which must be focused with respect to an objective for observing the fluorescence of these molecules. Since the visual field of the objective is very large and the depth of focus is very small, the substrate must be oriented with respect to the optical axis of the objective so that the entire surface occupied by molecules can be detected with uniform quality. The fluorescence is observed through the substrate, so that the back of the substrate which is populated by molecules, rather than the front side of the substrate, must be focused and oriented.